1. Field of the Invention
The present invention relates to an electric lamp employing a light-transmitting lamp vessel in which a light source is arranged, the electric lamp employing a light-absorbing medium exhibiting a spectral transition in the visible range, where at least a part of the lamp vessel being provided with an interference film.
Such lamps are used in automotive applications, for example as a (halogen) headlamp which, in operation, emits yellow light, as an amber-colored light source in indicators (also referred to as vehicle signal lamps) or as a red-colored light source in brake lights. Such electric lamps are also used for general illumination purposes. The electric lamps are further used in traffic and direction signs, contour illumination, traffic lights, projection illumination and fiber optics illumination. Alternative embodiments of such lamps employ lamps wherein the color temperature is increased by a suitable combination of a light-absorbing coating and an interference film.
2. Description of the Related Art
In a known electric lamp, an interference film reflecting blue light is provided on the lamp vessel of a (halogen) headlamp, which lamp, in operation, emits yellow light. Besides, a light-absorbing medium for absorbing the blue light reflected by the interference film is provided on an outer surface of the lamp vessel, between the lamp vessel and the interference film.
A drawback of the known lamps is that the appearance of the lamp is insufficiently color-neutral.
It is an object of the invention to provide an electric lamp to obviate the aforementioned drawback with the prior art.
This object is achieved, in accordance with the invention, in that the light-absorbing medium exhibits a comparatively steep spectral transition in the visible range.
To achieve this, the electric lamp in accordance with the present invention comprises a light-absorbing medium wherein a spectral transmission T of light transmitted by the light-absorbing medium changes from Txe2x89xa60.15 to Txe2x89xa70.75 in a wavelength range having a width xcexxe2x89xa675 nm.
The application of a light-absorbing medium having such a comparatively steep spectral characteristic in an electric lamp whose lamp vessel is provided with an interference film results in an electric lamp having an improved color-neutral appearance. In the known electric lamp, a light-absorbing medium, such as Fe2O3, is used whose spectral characteristic gradually changes over a comparatively large wavelength range. As a result, the spectral transmission of the combination of interference film and light-absorbing medium is subject to change over a comparatively large range of the visible region, which is accompanied by color effects, which are generally undesirable. The known electric lamp emits yellow light in operation, but in the off state, it is blue in appearance.
By applying, in accordance with the invention, a light-absorbing medium having a comparatively steep spectral characteristic, the spectral changes in the visible region are limited to a comparatively small wavelength range, which is favorable for obtaining a color-neutral appearance of the electric lamp. In operation, the electric lamp in accordance with the invention emits light of the desired color. In the offstate, the electric lamp in accordance with the invention is color-neutral in appearance.
Preferably, in the above-mentioned wavelength range, the spectral emission of the light transmitted by the light-absorbing medium changes from Txe2x89xa60.10 to Txe2x89xa70.80. Preferably, there is a change in the spectral transmission T of light transmitted by the light-absorbing medium in a wavelength range having a width xcexxe2x89xa650 nm. The color-neutral appearance of the electric lamp is better as the spectral transition in the light-absorbing medium is steeper. It is particularly favorable for the change in spectral emission in the light-absorbing medium to occur in a wavelength range from 30 to 40 nm.
In a preferred embodiment, the electric lamp is provided with an interference film having an at least substantially flat reflection spectrum over at least substantially the entire visible region. To achieve this, the electric lamp in accordance with the invention is characterized in that the variation in the reflection R of the interference film in the wavelength range from 400xe2x89xa6xcexxe2x89xa6690 nm is less than 10%.
In this preferred embodiment, within the given variation range, the reflection spectrum exhibits no negligible peaks or dips in the relevant range of the visible spectrum. As the reflection by the interference film is uniform throughout the visible region, the reflection is at least substantially independent of the wavelength in the relevant part of the visible region. The interference film in accordance with the invention will reflect all colors in the visible spectrum in the same manner and provides the electric lamp with a color-neutral appearance.
In the known electric lamp, a combination of a light-absorbing medium and an interference film is applied, said interference film comprising a so-called step filter. The term xe2x80x9cstep filterxe2x80x9d is to be taken to mean in the description of the current invention that the reflection spectrum exhibits a comparatively sharp spectral transition (from R≈100% to Rxe2x89xa610%) in a comparatively narrow wavelength range (xe2x89xa620 nm). The positioning of the spectral transition of this step filter is very sensitive to process variations. Small variations readily lead to a shift of the spectral transition, as a result of which the known electric lamp no longer meets legal requirements. In the known electric lamp, it is further necessary to apply an interference film having a comparatively high reflection in the relevant spectral range, thus rendering necessary a stack of comparatively many optical layers. These high reflection values are necessary to sufficiently enhance the comparatively small effect of the comparatively thin light-absorbing medium. In addition, the optical layers in the interference film of the known electric lamp must be at least substantially absorption-free in order to realize the high reflection values of the step filter. In the known electric lamp, the spectral transition of the step filter lies in a wavelength range from approximately 530 to approximately 540 nm.
It is to be noted further that the visible region globally comprises the wavelength range from 380xe2x89xa6xcexxe2x89xa6780 nm. Taking into account the sensitivity of the human eye and the fact that the eye sensitivity curve decreases rapidly at the edges of the visible region, it is sufficient, in practice, if the reflection spectrum in the electric lamp in accordance with the invention is at least substantially flat in the wavelength range from 400xe2x89xa6xcexxe2x89xa6690 nm. In a preferred embodiment of the electric lamp, the variation of the reflection R of the interference film in the wavelength range from 380xe2x89xa6xcexxe2x89xa6780 nm is less than 10%. Experiments have shown that a 5-10% variation in the reflection of the interference film can be readily achieved throughout the visible region.
A preferred embodiment of the electric lamp in accordance with the invention is characterized in that the reflection R of the interference film lies in the range from 0.50xe2x89xa6Rxe2x89xa60.90. In this preferred embodiment, the interference film has a metallic or silvery appearance. As a result thereof, the electric lamp in accordance with the invention can very suitably be used as an indicator lamp for automotive applications. Statutory regulations define a range, in the 1931 C.I.E. color triangle known to those skilled in the art, for the color point of the light emitted by such indicator lamps. A suitable combination of a light-absorbing medium and an interference film applied to an outside surface of the lamp vessel enables the appearance of the electric lamp to be changed. This particularly enables a distinction to be made between the appearance of the electric lamp in the off state and the color of the light emitted by the electric lamp during operation. The aim is, in particular, to provide an electric lamp which, in operation, emits a certain color, for example a so-called amber-colored or red-colored electric lamp, while, in the off state, the electric lamp has an at least substantially color-neutral appearance.
In vehicles it is desirable, for esthetical reasons, to provide indicator lamps and brake lights with a color-neutral appearance. Only when the electric lamp is activated, it shows the desired color, whereby the color point of the light emitted by the electric lamp meets statutory regulations. Moreover, in vehicles there is a tendency to accommodate amber-colored indicator lamps in the same reflector as the headlamp instead of in a separate reflector. In addition, the aim is to use luminaires in vehicles, which are provided with so-called xe2x80x9cclear coversxe2x80x9d, i.e., an observer situated outside the vehicle can directly see the indicator lamps or brake lamps in the luminaire. For reasons of safety, it is important that, apart from a color-neutral appearance, such indicator lamps are at least substantially free of coloring in reflection at light which is (accidentally) incident on the electric lamp. If, for example, sunlight or light originating from on-coming traffic is incident on a headlamp of a vehicle comprising an indicator lamp, the appearance of said headlamp, in reflection, should be at least substantially colorless or, in reflection, said lamp should emit at least substantially no color. Otherwise, this might confuse other road users and give rise to unsafe and/or undesirable situations.
In reflection, the spectral characteristic of the electric lamp in accordance with the invention differs from the spectral characteristic in transmission. In transmission, the light emitted by the electric lamp meets statutory regulations with respect to the color point, while, in reflection, the electric lamp is color-neutral, the appearance of the electric lamp being, for example, silvery. The current invention applies, in particular, to indicator lamps and brake lights of vehicles.
A synergetic effect is achieved using an electric lamp comprising a combination of a light-absorbing medium with a steep transition and an interference film giving the electric lamp a color-neutral appearance. In addition, the presence of the interference film may increase the stability of the light-absorbing medium in that the interference film serves as an oxygen barrier for the light-absorbing medium. Moreover, the interference film can counteract loss of color of the light-absorbing medium under the influence of external UV light, for example by a suitable material choice, a suitably chosen band gap (for example TiO2) or as a result of the fact that the interference film also reflects UV light. Experiments have shown that the adhesion of the combination of light-absorbing medium and interference film on the lamp vessel of the electric lamp is satisfactory and not, or hardly, subject to change during the service life. During the service life of the electric lamp in accordance with the invention, no visible delamination of the applied coatings is detected.
A further advantage of the application of an electric lamp comprising a combination of a light-absorbing medium with a steep transition and an interference film giving the electric lamp a color-neutral appearance, is that the spectral characteristic of the light-absorbing layer is less sensitive to variations in the location of the spectral transition in the light-absorbing layer. This implies that the spectral characteristic of the light-absorbing layer is less sensitive to variations in the thickness and/or the concentration of the light-absorbing medium.
An embodiment of an electric lamp in accordance with the invention is characterized in that a wall of the lamp vessel comprises the light-absorbing medium. Light-absorbing media can be readily incorporated in the wall of the lamp vessel, which is made, for example, from glass, such as quartz glass or hard glass, or from a translucent ceramic material. In this embodiment, the interference film is preferably directly applied to a side of the wall of the lamp vessel facing away from the light source. As the light-absorbing medium is provided in the wall of the lamp vessel and the interference film, light, which is reflected by the interference film, passes the light-absorbing medium twice, which leads to a further improvement of the effectiveness of the absorption process. In addition, light which is reflected to and fro between the interference film on both sides of the lamp vessel passes the light-absorbing medium twice at each reflection.
An alternative embodiment of the electric lamp in accordance with the invention is characterized in that the light-absorbing medium comprises a light-absorbing layer which is situated between the lamp vessel and the interference film. As the light-absorbing medium is arranged between the outside surface of the lamp vessel and the interference film, light, which is reflected by the interference film, passes the light-absorbing medium twice, which leads to a further improvement of the effectiveness of the absorption process. In addition, light which is reflected to and fro between the interference film on both sides of the lamp vessel passes the light-absorbing layer twice at each reflection.
A thickness tabs of the light-absorbing layer preferably lies in a range from 5 nmxe2x89xa6tabsxe2x89xa65000 nm. If the thickness of the light-absorbing layer is smaller than 5 nm, absorption hardly takes place and the intended shift of the color temperature is insufficiently achieved. If the thickness of the layer exceeds 5 xcexcm, too much light is absorbed, which adversely affects the lumen output of the electric lamp. A light-absorbing layer having a thickness of 1.5xe2x89xa6tabsxe2x89xa62 xcexcm is very suitable. The desired layer thickness is also prompted by the concentration of the pigment in the light-absorbing coating.
A preferred embodiment of the electric lamp is characterized in that
the light-absorbing coating comprises a network, which can be obtained by converting an organically modified silane by a sol-gel process,
said organically modified silane being selected from the group formed by compounds of the structural formula RISi(ORII)3,
RI comprising an alkyl group or an aryl group,
and RII comprising an alkyl group.
By making the light-absorbing layer from a network comprising an organically modified silane as the starting material, an optically transparent, non-scattering, light-absorbing coating is obtained which is capable of resisting temperatures up to 400xc2x0 C. By using an organically modified silane in the manufacture of the network, a part of the RI groups, the alkyl or aryl groups, remains in the network as an end group. As a result, the network does not comprise four network bonds per Si atom, but less than four network bonds per Si atom. In this manner, for example, a network is obtained comprising, on average, approximately three network bonds per Si atom. In spite of the fact that the network is partly composed of said alkyl or aryl groups, a network is obtained whose density is at least substantially equal to that of the customary silica network. Unlike the customary silica network, a network which is partly composed of said alkyl or aryl groups has a greater elasticity and flexibility. As a result, it becomes possible to manufacture comparatively thick light-absorbing coatings.
Preferably, the RI group comprises CH3 or C6H5. These substances have a comparatively good thermal stability. A network comprising methyl or phenyl groups enables thicker coating layers to be obtained. Experiments have further shown that coatings, wherein methyl or phenyl groups are incorporated in a network, are stable to a temperature of at least 350xc2x0 C. Said groups form end groups in the network and remain part of the network at said higher temperatures. At such a comparatively high temperature load on the light-absorbing coating, no appreciable degradation of the network occurs during the service life of the electric lamp.
Preferably, the RII group comprises CH3 or C2H5. Methyl and ethyl groups are particularly suitable because methanol and ethanol are formed in the hydrolysis, which substances are compatible with the pigment dispersion and evaporate comparatively easily. In general, the methoxy groups (xe2x80x94OCH3) react more rapidly than the ethoxy groups (xe2x80x94OC2H5) which in turn react more rapidly than (iso)propoxy groups (xe2x80x94OC3H7). For a smooth hydrolysis process, use is advantageously made of RII groups which are not too long.
Particularly suitable starting materials for the manufacture of the network in accordance with the invention are methyltrimethoxysilane (MTMS), wherein RI=RII=CH3, methyltriethoxysilane (MTES), wherein RI=CH3 and RII=C2H5, phenyltrimethoxysilane (PTMS), wherein RI=C6H5 and RII=CH3, and phenyltriethoxysilane (PTES), wherein RI=C6H5 and RII=C2H5. Such starting materials are known per se and commercially available.
A preferred embodiment of the electric lamp is characterized in that the light-absorbing medium has an amber-colored or red-colored transmission. Electric lamps which, in operation, emit amber-colored light can particularly suitably be used as an indicator lamp in vehicles. Electric lamps which, in operation, emit red light are particularly suitable as brake lights in vehicles.
The choice of selectively light-absorbing layers is limited by the requirement which, in accordance with the invention, is to be met by the steepness of the change of the spectral transmission of the light-absorbing medium. The choice of selectively light-absorbing layers is further limited by the thermal requirements to be met by such a light-absorbing layer. Said thermal requirements include the durability of the light-absorbing medium during the service life and the resistance to changing temperatures of the lamp vessel.
Preferably, the light-absorbing medium has an amber-colored transmission. A particularly suitable light-absorbing medium is CHROMOPHTAL yellow, chemical formula C22H6C18N4O2 and C.I. (constitution number) 56280. This organic dye is also referred to as xe2x80x9cC.I.-110 yellow pigmentxe2x80x9d, xe2x80x9cC.I. pigment yellow 137xe2x80x9d or Bis[4,5,6,7-tetrachloro-3-oxoisoindoline-1-ylidene)-1,4-phenylenediamine. An alternative light-absorbing medium having an amber-colored transmission is yellow anthraquinone, chemical formula C37H21N5O4 and C.I. 60645. This organic dye is also referred to as xe2x80x9cFilester yellow 2648Axe2x80x9d or xe2x80x9cFilester yellow RNxe2x80x9d, chemical formula 1,1xe2x80x2-[(6-phenyl-1,3,5-triazine-2,4diyl)diimino]bis-.
In an alternative embodiment, the light-absorbing medium has a red-colored transmission and comprises, by way of example, xe2x80x9cCHROMOPHTAL red A2Bxe2x80x9d with C.I. 65300. Said organic dye is alternatively referred to as xe2x80x9cpigment red 177xe2x80x9d, dianthraquinonyl red or as [1,1xe2x80x2-Bianthracene]-9,9xe2x80x2,10,10xe2x80x2-tetrone, 4,4xe2x80x2-diamino-(TSCA, DSL).
An embodiment of the electric lamp in accordance with the invention is characterized in that the interference film comprises layers of, alternately, a first layer of a material having a comparatively high refractive index and a second layer of a material having a comparatively low refractive index. The use of two materials simplifies the provision of the interference film. In an alternative embodiment, at least a third layer material is applied having a refractive index between that of the first layer and the second layer.
A preferred embodiment of the electric lamp in accordance with the invention is characterized in that the second layer of the interference film comprises predominantly silicon oxide, and the first layer of the interference film comprises predominantly a material having a refractive index which is high as compared to a refractive index of silicon oxide. Layers of silicon oxide can be provided comparatively readily using various deposition techniques.
Preferably, the first layer of the interference film comprises a material chosen from the group formed by titanium oxide, tantalum oxide, zirconium oxide, niobium oxide, hafnium oxide, silicon nitride and combinations of said materials. Preferably, the material of the first layer of the interference film predominantly comprises niobium oxide or silicon nitride.
Preferably, the interference films are Nd2O5/SiO2 type films, Ta2O5/SiO2 type films or mixtures thereof and comprise, preferably, at least 5 and at most approximately 17 layers. As a result of the comparatively small number of layers, the manufacturing costs of such an interference film are comparatively low.
The light source of the lamp may be an incandescent body, for example in a halogen-containing gas, or it may be an electrode pair in an ionizable gas, for example an inert gas with metal halides, possibly with, for example, mercury as a buffer gas. The light source may be surrounded by an innermost gastight envelope. It is alternatively possible, that the outermost envelope surrounds the lamp vessel.
The interference film and the light-absorbing layer may be provided in a customary manner by, for example, vapor deposition (PVD: physical vapor deposition) or by (dc) (reactive) sputtering or by a dip-coating or spraying process or by LP-CVD (low-pressure chemical vapor deposition), PE-CVD (plasma-enhanced CVD) or PI-CVD (plasma impulse chemical vapor deposition). The light-absorbing layer on the outer wall of the lamp vessel is preferably applied by spraying. If the light-absorbing medium forms part of the wall of the lamp vessel, then this medium is generally provided in the wall in the course of the manufacture of the lamp vessel.
It has been found that the combination of absorbing medium and interference film of the electric lamp in accordance with the invention substantially preserves its initial properties throughout the service life of the electric lamp.